There is used a surface acoustic wave device as an acoustic wave device that has an IDT (Interdigital Transducer) formed with a comb electrode on a surface of a piezoelectric substrate and uses an acoustic wave excited with an implied electrical power to the comb electrode. The acoustic wave device such as the surface acoustic wave device is being widely used for a circuit treating a radio signal such as a band pass filter for transmission, a band pass filter for reception or an antenna duplexer.
There is developed a WLP (Wafer Level Package) technology for downsizing a package to a size of an acoustic wave element in order to downsize a surface acoustic wave device. An IDT 12 and an electrode pad 13 are formed on a wafer-shaped piezoelectric substrate 10, with reference to FIG. 1 of Japanese Patent Application Publication No. 2006-352430. A first insulating layer and a second insulating layer are formed on the piezoelectric substrate 10 so as to have a through-hole on the electrode pad 13. It is therefore possible to electrically couple an upper face of the insulating layer and the IDT 12 via the through-hole.
There is, however, a problem that adhesiveness is not high between a resin portion acting as the first insulating layer and the electrode pad.